PL Parameters

For each safety-related part of the control system and/or the combination thereof that performs a safety function, the performance level (PL) must be determined (evaluated) by the estimation of the following principal aspects: 1) Category (Structure), 2) DC, 3)MTTFd, and 4) CCF.

Category

DC (Diagnostic Coverage)

Denotation

Range

None

DC < 60%

Low

60% ≤ DC ＜ 90

Medium

90% ≤ DC ＜ 99%

High

99% ≤ DC

DC is a measure of the effectiveness of the diagnostics, which may be determined as the ratio between the failure rate of detected dangerous failures and the failure rate of total dangerous failures. DC can exist for the whole or parts of a safety-related system. The four denotations shown in the table to the right are provided in ISO 13849-1.

MTTFd

Denotation

MTTFd

Low

3 years ≤ MTTFd < 10 years

Medium

10 years ≤ MTTFd < 30 years

High

30 years ≤ MTTFd < 100 years

MTTFd (Mean time to dangerous failure) is an expectation of the mean time to dangerous failure on the whole or part of a safety-related system. The MTTFd is given for each channel, such as “I” (Input device), “L” (Logic), and “O” (output device). The three denotations shown in the table to the right are provided in ISO 13849-1.

CCF

The CCF (Common Cause Failure) relates to the failure of different items, resulting from a single event, where the failures are not consequences of each other.
ISO 13849-1 provides a scoring process and quantification of measures against CCF. The total score must be 65 or better.

Determination of PL

(1) Category (five types: B, 1, 2, 3, and 4)

(2) MTTFd (three types: high, medium, and low)

(3) DCavg (four types: high, medium, low, and none)

(4) CCF (two types: ≥65 points and <65 points)

The following table can be used for determination of PL based on the above parameters.